Safety control system for fuel burners



- 1949 c. v. HILDEBRECHT SAFETY CONTROL SYSTEM FOR FUEL BURNERS Filed April 18, 1947 gi N W W 20.53% I mu lZHaZdeb/ C/Ji Patented Dec. 13, 1949 SAFETY CONTROL SYSTEM FOR FUEL BURNEBS Charles V. Hildebrecht, Chicago, Ill., asslgnor to Milwaukee Gas Specialty Company, Milwaukee, Wis., a corporation of Wisconsin Application April 18, 1947, Serial No. 742,337

9 Claims. 1

This invention relates, in general, to a burner control system, and has particular relation to burner control systems employing means for interrupting the supply of fuel to a main burner in the event a pilot burner is extinguished.

While the particular system which I shall describe hereinafter in connection with the drawing is, in general, of the type in which an electric circuit for an electroresponsive valve in the fuel conduit has contacts under the control of a relay in the control circuit, it is to be understood that the invention in its broader aspects'is not limited to this particular type of system, but may be embodied in other types of systems as suitable or desired.

Prior systems of the type with which the present invention is concerned have proposed the use, in position, for example, where it is subject to the heat of a pilot burner, of a thermistor which has a negative temperature coefficient of resistivity and operates when heated by the pilot burner to pass sufiicient current to maintain an electroresponsive valve in the fuel conduit open when the pilot burner is burning and the circuit, including the electroresponsive valve, is otherwise closed, and to reduce the current so that the electroresponsive valve operates to closed position when the pilot flame is extinguished.

In these prior systems however, the thermistor has usually been subjected, for example, to the heat within the burner compartment and/or to the heat of the main burner or other ambient temperature. As a result, prompt actuation of the valve to closed position when the pilot flame is extinguished is not assured because the ambient temperature may heat the thermistor an amount to pass suflicient current to maintain the electroresponsive valve in open position for a considerable period of time after the pilot flame is extinguished.

With the valve in the fuel conduit open and the pilot flame extinguished, fuel may flow in considerable volume to the main burner. As a result the burner compartment may be filled, for example, with a highly explosive gas-air mixture, thus presenting the possibility of serious and disastrous explosion when it is attempted to relight the pilot burner. These things can happen and have happened in using defective so-called safety devices, and, profiting by experience, the requirements of the American Gas Association that such devices be safe have become increasingly stringent.

In the copending application of Philip B. Marowned by the same assignee of this application,

now Patent No. 2,491,725, granted Dec. 13, 1949,

there is provided an improved burner control system in which the foregoing problems are overcome, and, more particularly, a burner control system having a first thermistor type of device subject to the heat of a pilot flame, and a second thermistor type of device responsive to the ambient temperature and operable to disable the relay or other means for maintaining the valve open so that it will operate to closed position to shut off the supply of fuel when, for example, at the time the pilot light is extinguished, the ambient temperature is such that it might otherwise maintain the valve open or retard its operation to closed position.

One of the main objects of the present invention is to provide in a system of the class described means subject to the heat of the pilot flame and connected to prevent disabling of the valve controlling means by said second device when the pilot burner is lighted.

Another object of the invention is to provide a system wherein the means subject to the heat of the pilot flame for preventing disabling of the valve controlling means by said second device when the pilot burner is lighted has a positive temperature coeflicient of resistivity.

Another object of the invention is to provide a system wherein said second device is connected in shunt relative to the valve controlling means and wherein the means for preventing disabling of the valve controlling means by the second device is connected in series with said second device.

Another object of the invention is to provide a burnerv control system of the character described having various features of novelty and advantages, and which is particularly characterized by its simplicity in construction, its economy in manufacture and operation, its effectiveness in use, and the absence of moving contacts and the like in the devices subjected to the heat of the pilot flame and to the ambient temperature.

Further objects and advantages of the invention will appear from the following detailed description, taken in connection with the accompanying drawing in which the single figure is a diagram showing schematically the parts and circuit connections of one form of system embodying the present invention.

Referring now to the drawing, the burner shown at I is a main burner. It may be the main burner for a room or space heater, or it may be vln, Serial No. 728,893, filed February 15, 1947, the burner for a water heater or for a floor furme. an oven burner, one or more top burners for a gas range, or any other burner, and of any suitable construction.

A fuel supply pipe or conduit 2 leads to the burner I for the delivery of gaseous or other fuel thereto, for example, through a mixing chambet 2 to which air is admitted through adjustable air inlets (not shown), as well understood in the art. The flow of gas through the conduit 2 is controlled by a valve 4 of any suitable electroresponsive or electrically operated type. For purposes of illustration, the valve 4 may be a solenoid valve operated to open position by the flow of current through its coil 6, and to closed position by a spring I.

The pilot burner 8, which is located in juxtaposition to the main burner l to maintain a pilot flame 9 for igniting the main burner is supplied with gaseous fuel by a pilot supply pipe [0. The pilot supply pipe I may be connected, for example, to the pipe 2 anterior of the valve I, or through'this valve, or in some cases posterior of the valve, or to any other source offuel supply.

Power for operating the valve 4 is supplied from a suitable source, for example the line wires l2 of a source of alternating current such as the type of alternating current line which averages about 115 volts. The circuit through the coil 6 of the valve 4 is controlled, for example, by a relay I3 having a coil l4 and a pair of contacts 15. One contact I5 is connected in circuit with one of the line wires I2. The other contact is connected by a conductor IE to one side of the coil 6, and the other side of the coil 6 is connected in circuit with the other line wire [2.

A thermistor I8 is positioned so as to be heated by the pilot flame l9 when the pilot flame is burning. The thermistor l8, which is preferably not positioned directly in the flame, has a negative temperature coeflicient of resistivity, i. e., it is a non-conductor, or relatively a nonconductor, when not subjected to the heat of the pilot flame or a corresponding temperature, and changes to an electric conductor when heated by the pilot flame or a corresponding temperature.

One side of the thermistor i8 is connected by a conductor to one side 2| of a suitable voltage source.' A conductor 22 connects the other side of the thermistor l8 through an adjustable resistor 22, and the coil H of the relay I3 to the other side 24 of the voltage source. The coil M of relay l3, adjustable resistor 23', and thermistor II are thus connected in series in an electric circuit leading, for example, from one side of the voltage source through the thermistor, adjustable resistor, and relay coil to the other side 24 of the voltage source.

The adjustable resistor 23 is preferably ad justed to pass just enough current to cause the relay II to hold the contacts l5 closed as lon as the thermistor I8 is heated by the pilot flame and there is suificient diiferential in the pilot flame temperature and the ambient temperature, as will presently appear.

A second thermistor 26 having a negative coefllcient of resistivity is connected in a shunt circuit around the coil ll of the relay i3, and the shunt circuit includes a resistor 21 in series with the thermistor 28. Means, for example, in the form of a resistor 28, having a positive temperature coefllcient of resistivity, is also connected into the shunt circuit and in series with the second thermistor 26 or equivalent device. The

4 resistor2lispositionedsoastobeheatedbythe pilot flame ll when the pilot flame is burning. This device 22, which is preferably not positioned directly in the flame-although this may varyhas, as already stated, a positive temperature coemcient of resistivity, i. e., it is an electric conductor when not subjected to the heat of the pilot flame or a corresponding temperature, and changes to a non-conductor, or relatively a nonconductor, or has its conducting properties reduced when heated by the pilot flame or a corresponding temperature.

' The shunt circuit comprises a conductor 22 connecting one side of the resistor 22 to the conductor 22, through resistor 21, to one side of relay coil H, a conductor 22 connecting the other side of the resistor 28 to the second thermistor 26 or equivalent device, and a conductor 28' connecting the other side of the thermistor 26 to the conductor 2|.

Thethermistor 2G is positioned so as to be subjected to the ambient temperature, which may be the heat within the compartment (not shown) for the main burner and/or the heat of the main burner or other. ambient temperature. As previously stated, the adjustable resistor 22 is preferably adjusted to pass just enough current to cause the relay l3 to'maintain the contacts I5 closed and thereby to maintain the valve 4 open as long as the thermistor I8 is heated by the pilot flame and there is a suflicient diflerential in the pilot flame temperature and the ambient temperature. The desired balance can be obtained by connecting suitable milliameters in the circuits for the respective thermistors i8 and 22 and noting the readings in adjusting the resistor 23.

The resistor or other device 28 is of such character as to prevent by-passing enough current around the coil H of relay I3 by thermistor 28 to permit contacts l5 to separate as long as resistor 28 is heated by the pilot flame l9, i. e., as long as the pilot flame is burning. In other words, the resistor 28 constitutes means positioned to be subjected to the heat of the pilot flame and connected to prevent deenergization of the electromagnet means 6, I of the valve controlling means by the thermistor 26 when the pilot burner is lighted.

In operation, with the heat of the pilot flame heating the thermistor l8, this thermistor l8 passes current through the relay coil l4 suillcient to close contacts I 5. The voltage across the line wires I2 is thus applied to the coil 6 of the valve 4, opening this valve and holding it open for the supply of fuel to the burner l as long as this condition exists.

If, when the pilot flame isextinguished, the ambient temperature rises an amount to heat the thermistor I 8 to the temperature to which said thermistor I8 is heated by the pilot flame, when ignited, such ambient temperature might, without the improvements herein described, heat the thermistor 18 an amount to pass/suflici'en't current to maintain the electro-responsiv valve 4 in open position for a considerable period of time after the pilot flame is extinguished or otherwise to open valve 4 when the pilot-flame is extinguished. However, with the present improvements, with the pilot flame extinguished the thermistor 26, when subjected to such ambient temperature, by-passes enough current around coil ll of relay i3 to permit the contacts IE to separate. This opens the circuit for the valve 4,

which operates to closed position, for example, by spring 1, to shut ofi the supply of fuel to the burner I.

In the combination described, the thermistors l8 and 26 having negative coeflicients of resistance decrease in resistance when the temperature is increased so as to pass more current when a voltage is applied to their circuits, and the thermistor 28, having a positive coeificient of resistance, increases in resistance when its temperature is increased so as to reduce any current flowing therein.

-Moreover, the energizing current required to actuate relay [3 will be of a certain value, and its holding current will be of a lower value; these values will depend in part upon the resistances of thermistors I8, 26, and 28 when heated, and cold (in this condition relay l3 will be deenergized).

The resistor 23 constitutes an adjustable protective device for the thermistor l8, and the resistor 21 constitutes a suitable protective device for the thermistor 26. These resistors constitute current limiting devices for preventing destructive overheating of the thermistors.

If during operation of the system with the thermistor l8 heated by the pilot flame sulficiently to maintain the valve 4 open and the ambient temperature is insufficient to heat the thermistor 26 suificiently to by-pass the current around the relay [3 so as to cause closing of the valve 4 and the pilot flame is extinguished, the following action occurs: the thermistor l8 immediately cools and becomes non-conducting, or sufficiently non-conducting, so that it will not pass sufiicient current to cause relay 13 to 'hold contacts I5 closed. These contacts then open the circuit for the valve 4 which operates to closed position, shutting ofi the supply of fuel to the burner I.

To close the circuit which opens the valve 4 and maintains it open, the pilot burner is lighted, and the flame I9 issuing therefrom heats the thermistor I8 and causes it to pass suflicient current through the relay I3 to close contacts Hi. This energizes the coil 6 of the valve 4', which thereby operates to open position to pass fuel to the burner I. It is thus apparent that no passage of gas to the burner I can occur unless the pilot flame is burning; also that thermistor 26 prevents the ambient temperature from holding valve 4 open after the pilot flame is extinguished, whereas resistor 28 prevents closing of valve 4 by thermistor 26 while the pilot flame is burning.

It is to be understood that the electric circuit for controlling the valve 4 may have a thermostat, limit device, or other control means therein, in which case the valve 4will be caused to open and to be held open by the heat of the pilot flame on the thermistor [8 only when such thermostat, limit device, or other control is in position closing the circuit which includes the thermistor l8, and the thermistor 26 is not heated by the ambient temperature sufliciently .to by-pass the coil l4 of the relay I3.

The thermistors l8 and 26 may be of any suitable form. Such devices are well known, and will not be described in detail. Suflice it to state that the thermistor elements may be formed, for example, of ferricv oxide (F6203) with about of titanium dioxide (T102) of suitable particle size, or by mixing oxides with an organic binder and solvent, and applying suitable contacts, or otherwise as desired.

It is to be further understood that where I refer to the device 28 as a resistor, having characteristics known in the art, that this device may be in various other forms, such as a thermostat or the like, within the scope of the broader aspects of the present invention.

The embodiment of the invention shown in the drawing is for illustrative purposes only, and it is to be expressly understood that said drawing and the accompanying specification are not to' be construed as a definition of the limits or scope of the invention, reference being had to the appended claims for that purpose.

I claim:

1. In a system of the class described, in combination, a fuel conduit, an electroresponsive valve in said conduit, an electric circuit having means therein for controlling said valve, a pilot burner, a first member having a negative coefiicient or resistivity connected into said circuit and positioned to be subjected to the heat of the pilot flame so as to pass suflicient current to said valve controlling means to actuate said valve to open position and to maintain said valve open when the pilot burner is lighted, said member reducing the current to said valve controlling means to deenergize said electroresponsive valve for actuation thereof to closed position when the pilot flame is extinguished, a second member having a negative coefficient of resistivity positioned to be subjected to ambient temperature and connected to deenergize said electroresponsive valve for actuation thereof to closed position as the ambient temperature approaches the temperature to which said first member is subjected by the heat of the pilot flame, and means positioned to be subjected to the heat of the pilot flame and connected to prevent deenergization of said electroresponsive valve for actuation thereof to closed position by said second member when the pilot burner is lighted.

2. A system according to claim 1 wherein said last means has a positive temperature coefficient of resistivity.

3. A system according to claim 1 wherein said second member is connected in shunt relative to said valve controlling means and wherein said last means is connected in series with said second member.

4. A system according to claim 1 wherein said second member is connected in shunt relative to said valve controlling means and wherein said last means has a positive temperature coefiicient of resistivity and is connected in series with said second member.

5. A system according to claim '1 wherein said second member is connected in shunt relative to said valve controlling means and wherein said first circuit and said shunt circuit have resistors constituting current limiting devices for preventing destructive overheating of said first and second members.

6. A system according to claim 1 wherein said valve controlling means comprises a relay controlling contacts in the circuit for said electroresponsive valve.

'7. In a system of the class described, in combination, a fuel conduit, a pilot burner, a circuit connected to a voltage source and having therein a relay and a first thermistor subject to the heat of the pilot burner, an electroresponsive valve in said conduit, a circuit connected to a voltage source and having therein said electi cresponsive valve and contacts for said relay held closed by said relay when said thermistor is heated by the pilot flame, said thermistor having a negative temperature coeflicient of resistivity. and operable 7 when the pilot flame is extinguished to reduce the current flow through said relay to deenergize said electroresponsive valve for actuation thereof to closed position, a circuit connected in shunt relative to said relay, a second thermistor in said shunt circuit and positioned to be subjected to ambient temperature, said second thermistor having a negative coemcient of resistivity and adapted, when the ambient temperature approaches the temperature to which said first thermistor is subjected by the heat of the pilot flame, to shunt enough current around said relay to deenergize said electroresponsive valve for actuation thereof to closed position, and means positioned to be subjected to the heat of the pilot flame and connected in series with said second thermistor to prevent deenergization of said electroresponsive valve for actuation thereof to closed position by said second thermistor when the pilot burner is lighted.

8. In apparatus of the class described, in combination, a burner, an electroresponsive device, a control circuit for said device having therein a flrst thermistor with a negative coeiflcient of resistivity in juxtaposition to the burner flame so as to pass sufficient current to energize said electroresponsive valve when the burner is lighted, said thermistor reducing the current in said control circuit sufliciently to deenergize said electroresponsive device when the burner is extinguished, a second thermistor with a negative coefllcient of resistivity positioned to be subjected to ambient temperature and connected to deenergize said electroresponsive valve as the ambient temperature approaches the temperature to which said first thermistor is subjected by the burner flame, and means having a positive coeificient of resistivity and positioned to be subjected to the heat of the burner flame and connected to prevent deenergization of said electroresponsive device by said second thermistor when the burner is lighted.

9. In apparatus of the class described, in combination, a main burner, a fuel supply line therefor, an electroresponsive valve in said fuel supply line, a pilot burner in juxtaposition to said main burner, a pair of thermistors each having a negative coefficient of resistivity, with one positioned to be subjected to the heat of the pilot flame to energize said valve when the pilot flame is lighted and the other positioned to be subjected to ambient temperature to deenergize said valve as the ambient temperature rises in amount to heat the thermistor subjected to the pilot flame to the temperature to which it is heated by the pilot flame, and means having a positive coemcient of resistivity and positioned to be subjected to the heat of the pilot flame and operable to render the thermistors subject to ambient temperature ineifectual to deenergize said valve as long as said pilot burner is lighted.

CHARLES V. HILDEBRECHT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

